Since the growth and nourishment of established tumors is dependent on their induction of host microvasculature, we wish to study the effects of radiation on the sensitivity of replicating endothelial cells in culture and in a transplantable hemangioendothelioma in 129/J strain mice. After determining radiation dose-response curves for endothelial cell division, protein synthesis, chromosomal aberration, and ultrastructural changes, we will evaluate the capacity of radiosensitizing agents and radioprotective agents to modify the responses. Results from these studies may facilitate the design of therapeutic regimens which enhance the destruction of replicating endothelial cells of microvasculature required for tumor growth but diminish the damage to the non-proliferating endothelial cells in the microvasculature of normal tissues adjacent to the tumors and within the radiation fields. Moreover, results of this work will contribute to our understanding of the biological processes of tissue damage and repair.